I don't know if you feel comfortable about talking to the "new sweetie" but it might help to find out from the 'horse's mouth' what she suspects. If the common sweetie molested her kid, chances are he molested your kids too. If you both go to the police with your suspicions and ask for victim's assistance, it will carry more weight than if you blame each other.

Be sure to talk to your kids about sexual abuse and make sure they know that it is never the child's fault.